Added basics of Banach algebras.

refs.bib

@@ -122,3 +122,13 @@
   year={1978},
   publisher={Springer}
 }
+@book{Zhu,
+  title={An Introduction to Operator Algebras},
+  author={Zhu, K.},
+  isbn={9780849378751},
+  lccn={93007172},
+  series={Studies in Advanced Mathematics},
+  url={https://books.google.ca/books?id=XHLj7bz8hOIC},
+  year={1993},
+  publisher={Taylor \& Francis}
+}